PART 5 A MONOGRAPH OF THE EXISTING CRINOIDS 639 



developed, cspecinlly on the ventral side and along the vestibule. Within the vestibule 

 a regular arrangement of the glandular cells is apparent. At the anterior end they 

 occupy the bottom of the furrow while the sides are occupied by simple cells seen in 

 sections as very conspicuous masses of nuclei. Proceeding downwards the nuclear 

 masses gradually pass do^vn into the furrow, narrowing the glandular space at the 

 bottom until it disappears entirely and the nuclear masses occupy the bottom. At 

 the same time new glandular masses appear along the sides of the furrow, these in their 

 turn again passing deeper down into the furrow narrowing the nuclear mass to a re- 

 stricted area in the middle of the furrow, while new nuclear masses appear along the 

 sides, these last, however, remaining less conspicuous. The epithelium of the vestibide 

 is distincth' ciliated. In sections the cuticle is seen to be perforated while a regular 

 layer of fine grains occurs just below it. 



The suctorial disk is large and deep. Its cells contain a finely granular substance. 

 A slimy mass fills the cavity of the disk, probably repi'escnting cilia destroyed by 

 preservation. The cuticular structure observed in the vestibule cannot be distinguished 

 here. There is no indication of an anterior tuft of cilia. 



Pigmentation begins to develop in the skin of the larva before it leaves the egg 

 membrane in the form of fine dark specks, which give the larva a faint grayish tint. 



The nervous system is fairly well developed. Mortensen was unable to make out 

 the details, but he says that there is no reason to doubt its complete conformity with 

 that in other crinoids. 



The vestibular invagination is remarkably narrow, deepest at the anterior end. 

 Closure begins at the posterior end and is completed while the embryo is still within the 

 egg membrane. In closing, the lumen is so nearly obliterated that in sections merely 

 a line is seen indicating the boundary between the epithelium of the roof and that of the 

 floor. The last vestige of the invagination is a very narrow opening in the form of a 

 deep canal at the anterior end just below the suctorial disk. According to Mortensen, 

 the walls of the vestibule must later separate, since eventually the vestibule acquires 

 the typical form. 



The hydrocoele has begun to differentiate, the 5 primary tentacles (or radial 

 canals) having appeared. The stone canal is forming, but has not yet opened into the 

 parietal canal. The hydropore is about to disappear, or has become entirely closed. 

 The anterior prolongation of the parietal canal is still distinct. 



There is but little change in the coelomic vesicles. The left or oral coelome has 

 developed two prolongations directed upward on the ventrolateral side so that in trans- 

 verse sections two small spaces appear at the side of the hydrocoele, its whole dorsal 

 portion being occupied by the right or aboral coelome. This has acquired a somewhat 

 complicated form because of a deep notch in the upper side of the stomach. 



The axial organ has begim to develop as a thickening of the coelomic epithelium 

 in the notch at the vertical mesentery. 



The entoderm still remains in the same histological condition as in the preceding 

 stage, the lumen being more or less indefinite or entirely obscured through the passage 

 inward of the yolk masses. The stomach is now somewhat lobed because of the notches 

 from the aboral coelome. One of these lobes, according to Mortensen, is probably 

 destined to form the intestine. 



Mortensen remarks that the fact that the ciliated bands of the larva are well 

 developed proves that the free-swimming stage has not been entirely obliterated; but 



